The UK is facing a "multidimensional threat" that is rapidly evolving "at a scale and pace we've not seen before", the director general of MI5 has warned.
Andrew Parker said the security service is operating at an unprecedented level and has seen "a dramatic upshift in threats this year".
"It's at the highest tempo I've seen in my 34-year career," he said. "Today there is more terrorist activity, coming at us more quickly, and it can be harder to detect.
"We're now running well over 500 live operations involving around 3,000 individuals known to be currently involved in extremist activity in some way.
"As well as those we are looking at today, risk can also come from returnees from Syria and Iraq and also the growing pool of over 20,000 individuals we've looked at in the past in our terrorism investigations.